(1) Field of the Invention
The present invention relates to a silver halide photographic light sensitive material (referred to as "photographic light sensitive material" hereinafter), and particularly to a photographic light sensitive material having and improved blackness of image silver, high sensitivity, and high covering power.
(2) Description of Prior Art
The covering power of a silver halide emulsion is a matter of great concern for emulsion makers, because the amount of silver required to maintain a given optical density by using an emulsion having a high covering power can be decreased. It is disclosed in U.S. Pat. Nos. 2,996,322, 3,178,282, 3,397,987 and 3,607,278, British Pat. No. 1,426,277, etc., that extremely high covering power can be obtained by mixing a surface sensitive emulsion having a high iodine content with an emulsion comprising small-sized grains which are internally fogged, in order to increase the covering power of a silver halide emulsion. It is also disclosed in U.S. Pat. Nos. 4,411,986, 4,434,226, 4,413,053, etc. that the covering power of developed silver can be greatly increased by using a tabular grain emulsion having a high aspect ratio and a small grain thickness. Furthermore, Japanese Patent Laid-Open No. 108,526/1983 discloses that an extremely high sensitivity and a covering power can be obtained by using so-called epitaxial tabular grains in which another silver salt is arranged in particular sites of tabular grains having high aspect ratios and of small thicknesses. It is known that the use of fine grains of a highly sensitive emulsion having a small average grain size, or tabular grains of a highly sensitive emulsion having a small average grain thickness other than those of the abovedescribed emulsion provides high covering power for silver, as compared with those of emulsions, which have a large average grain size and large average grain thickness.
The color tone of developed silver of emulsion grains, which grains provide silver with high covering power, depends, practically without exception, upon the grain size and grain thickness. However, the tone is yellowish and gives an unfavorable impression to the observer of the resulting picture image. This yellowishness is caused by the strong yellowish light produced by the increase in scattering of blue light due to the decrease in size and thickness of the developed silver generated by the decrease in size and thickness of the grains. Such a phenomenon is a serious problem when a fine grain emulsion (for example, an average grain size of 0.4.mu. or less) or tabular grain emulsion having a small grain thickness (for example, a grain thickness of 0.4 or less) is used as a silver halide emulsion. In order to resolve such a problem, a tone-regulating agent is generally used for regulating the color tone of the developed silver. For example, a certain kind of mercapto compound may be used for this purpose. However, application of such a conventionally known tone-regulating agent to an emulsion comprising silver halide grains having high sensitivity causes extreme desensitization, and thus, such an emulsion cannot be put into practice. Japanese Patent Laid Open No. 158436/1985 discloses a technique for improving the color tone of a silver image by including a fluorescent brightener in a sensitive material comprising a tabular silver halide grains having a grain size of 5 times or more the grain thickness. This technique is very effective for observation under a reflected light souce, but it is not effective for observation under transmission light, as is described in such an application.